Physiology of Hearing, Smell and Taste Flashcards
1
Q
What are the stages of hearing?
A
- First transduction: sound waves strike the tympanic membrane and become vibrations.
- The sound wave energy is transferred to the 3 bones of the middle ear, which vibrate.
- Second transduction: the stapes is attached to the membrane of the oval window. Vibrations of the oval window create fluid waves within the cochlea.
- Third transduction: the fluid waves push on the flexible membranes of the cochlear duct. Hair cells bend and release neurotransmitter.
- Fourth transduction: neurotransmitter release onto sensory neurons creates action potentials that travel through the cochlear nerve to the brain.
- Energy from the waves transfers across the cochlear duct into the tympanic duct and is dispiated back into the middle ear at the round window.
2
Q
What is the innervation of the external ear?
A
- Auricular branch of the vagus.
- Auriculotemporal branch of trigeminal.
3
Q
Describe the features of the tympanic membrane.
A
- Concave
- Shadow of the handle of the malleus
- 4 quadrants
- Safest quadrant is the antero-inferior quadrant
- Chorda tympani is in the postero-superior quadrant
- Triangular reflection of light in the Al quadrant (Politzer’s triangle)
- Rich neural innervation
4
Q
Describe the middle ear.
A
- Air filled cavity with ossicles, muscles and nerves.
- Ossicles that transmit the vibration from the tympanic membrane to the inner ear
- Malleus, incus, stapes
- Attached to the walls by ligaments
- Small muscles
- Tensor tympani
- Stapedius
- Chorda tympani
- Auditory / pharyngotympanic / Eustachian tube
- Mucous membrane continuous with the pharynx
- Supplied by glossopharyngeus
5
Q
Give the action and innervation of tensor tympani.
A
- Tensor tympani pulls the TM medially → increase the tension in response to loud noises → reduce the vibration of the tympanic membrane.
- Supplied by mandibular nerve.
6
Q
Give the action and innervation of stapedius.
A
- Stapedius pulls the base of the stapes away from the oval window.
- Protects the inner ear from injury from a loud noise.
- Supplied by facial nerve.
7
Q
Describe the pharyngotympanic tube.
A
- Walls are normally collapsed.
- Actively opened by the simultaneous contraction of the tensor veli palatini and salpingopharyngeus muscles.
- The tube is short and straight in children.
8
Q
Describe the inner ear.
A
- Bony labyrinth
- Vestibule
- Utricle
- Saccule
- Semicircular canals
- Ducts
- Cochlea
- Vestibule
- Membraneous labyrinth
- Perilymph
9
Q
Describe the ear’s mechanism for frequency detection.
A
- Structure of basilar membrane changes from short and stiff to long and floppy along the length of the cochlea.
- Resonant frequencies vary along the cochlea with high frequency at the base and low at the apex.
- When the basilar membrane vibrates at the resonant frequency, it absorbs all the kinetic energy of the wave and effectively stops it at that point.
10
Q
Describe the signal detection at the organ of corti.
A
- Upward deflection of the basilar membrane moves the inner and outer hairs laterally with respect to the tectorial membrane.
- 95% of the cochlear nerve endings terminate on the inner hair cells.
- Outer hair cells increase the sensitivity of inner hair cells.
- This can tune the cochlea by amplifying select frequencies.
11
Q
How are the stereocilia involved in signal transduction?
A
- Displacement of stereocilia in one direction opens K channels, and closes them in the other.
12
Q
Describe the auditory pathways.
A
- Hair cells of the organ of Corti generate an electrical signal.
- Peripheral extensions of the bipolar neurons at spiral ganglion synapse with hair cells of the Organ of Corti.
- Central extensions of bipolar neurons from the cochlear nerve (1st order).
- Cochlear nerve synapses at anterior and posterior cochlear nuclei.
- Central extensions of 2nd order neurons spilts up, with some travelling ipsilaterally, but most contralaterally up to the respective superior olivary nucleus.
- Lateral leminiscus (3rd order) ascend and synapse at inferior colliculus.
- 4th order neurons project to the medial geniculate nucleus of the thalamus where they synapse.
- 5th order neurons join the auditory cortex.
- Collaterals from the pathway project into the reticular formation and the vermis of the cerebellum causing arousal responses to noise.
- Secondary projections from the primary, and some from the thalamic association areas then go to the auditory cortex.
- Sound is relayed tonographically to these cortical areas, with lower frequencies to the anterior in most maps though there are variations.
13
Q
Describe how the ear detects the direction of sound.
A
- Volume
- Sound shadow
- Sound from one side hits the head, which then generates a sound shadow on the other side in which the volume is less. Comparison of signal intensities from both ears determines the ear closest to the sound.
- Sound lag
- Sound from a particular direction enters one ear before the other and so there is a slight delay between the sound arriving ipsilaterally at the auditory cortex, and that arriving contralaterally.
- Sound lag (which works at lower frequencies) is better at determining horizontal direction than sound shadow (which is good for high frequencies).
- Neither method detects front to back or above to below directionality.
- This is achieved by the folds in the pinna which changes the characteristic of sound coming from above compared to below etc.
14
Q
What are the causes of conduction deafness?
A
- A blockage in the outer ear.
- Infection in either the outer or inner ear.
- Ossification of the small bones in the middle ear.
- Rupture of the tympanic membrane.
15
Q
What are the causes of sensorineural deafness?
A
- Breakdown of the cochlea and associated mechanisms.
- Damage to the auditory nerve.
- Damage to the auditory cortex.
16
Q
Name the tests which differentiate between conduction and sensorineural hearing loss.
A
- Rinnes - air condution is better than bone conduction = positive test.
- Webber’s - tuning fork in middle of forehead, heard equally on both sides.
18
Q
Describe the taste buds and papillae of the tongue.
A
- Taste buds are located on the oral surface of the soft palate, the posterior wall of the oropharynx and the epiglottis.
-
Papillae
-
Vallate papilla
- Along sulcus terminalis
- Supplied by glossopharyngeal nerve
- More sensitive to bitter
- Fungiform papilla
- Most numerous
- Supplied by facial nerve
- Foliate papilla
- Poorly developed
-
Vallate papilla
21
Q
What are the components of the olfactory system?
A
- Olfactory epithelium in the olfactory region.
- Receptor cells (bipolar neurons)
- Axons that project through the base of the skull to the olfactory bulb.
- euronal tract to multiple olfactory destinations in the brain.
22
Q
Describe olfactory epithelium.
A
- Olfactory neuroepithelial cells have a life span of 40-60 days and regenerate from basal cells.
- Basal cells can be used as stem cells.
- Glandular goblet cells (Bowman glands) moisten the olfactory cells.
- Facilitate olfaction.
- Only about 2% of inhaled air comes in contact with the olfactory receptors.
- Sensitivity can be increased by forceful sniffing.
